allow readobjnam arg to be nonnull

Have it key on &do_random_str instead of NULL,
and modify makewish() in zap.c for the new protocol.

src/decl.c

@@ -1069,8 +1069,10 @@ decl_globals_init(void)
     gu.urace = urace_init_data;
 }
 
-/* fields in 'hands_obj' don't matter, just its distinct address */
+/* fields in 'hands_obj' and contents of 'do_random_str' don't matter,
+ * just their distinct addresses */
 struct obj hands_obj = DUMMY;
+char do_random_str = '\0';
 
 /* gcc 12.2's static analyzer thinks that some fields of gc.context.victual
    are uninitialized when compiling 'bite(eat.c)' but that's impossible;

src/objnam.c

@@ -4576,7 +4576,7 @@ readobjnam_postparse3(struct _readobjnam_data *d)
 
 
 /*
- * Return something wished for.  Specifying a null pointer for
+ * Return something wished for.  Specifying &do_random_str for
  * the user request string results in a random object.  Otherwise,
  * if asking explicitly for "nothing" (or "nil") return no_wish;
  * if not an object return &hands_obj; if an error (no matching object),
@@ -4588,7 +4588,7 @@ readobjnam(char *bp, struct obj *no_wish)
     struct _readobjnam_data d;
 
     readobjnam_init(bp, &d);
-    if (!bp)
+    if (bp == &do_random_str)
         goto any;
 
     /* first, remove extra whitespace they may have typed */

src/zap.c

@@ -6024,7 +6024,7 @@ makewish(void)
         if (++tries < MAXWISHTRY)
             goto retry;
         pline1(thats_enough_tries);
-        otmp = readobjnam((char *) 0, (struct obj *) 0);
+        otmp = readobjnam(&do_random_str, (struct obj *) 0);
         if (!otmp)
             return; /* for safety; should never happen */
     } else if (otmp == &nothing) {